If you are using a command to append a multiline to a tabular, it will stop working if the command has optional arguments. It will work without them:
\documentclass{standalone}
\newcommand{\mwe}[2][1]{ % remove [1]
\multicolumn{2}{l}{text}
}
\begin{document}
\begin{tabular}{ll}
\mwe[1]{1} % [1]->{1}
\end{tabular}
\end{document}
(Minimal working example, works when changed according to comments)
Error:
Misplaced \omit